Many applications of a holography technology that records an interference pattern of an object wave and a reference wave are proposed and implementation is being discussed. As one application of the holography technology, a holographic memory is proposed. In the holographic memory, light that is an object wave holding image information (referred to, hereinafter, as an “information beam”) is used. The information beam and a reference wave (also referred to as a reference beam) are overlapped within a recording medium. An interference pattern formed at this time is written in the recording medium, thereby recording information (Patent Literature 2(.
In the holographic memory, a region of the recording medium at which the information beam and the reference beam are irradiated, namely an address of the information to be recorded, is required to be identified. Therefore, it is proposed that a servo pit used for positioning and identifying the address is provided in the recording medium in advance. However, an unnecessary interference pattern may be recorded in an information recording layer and noise may be generated in a reproduction beam as a result of the information beam and the reference beam being diffused and reflected by the servo pit provided in the recording medium.
Therefore, in Patent Literature 1, a recording medium 101 that includes a filter layer 106 is proposed (FIG. 7). The filter layer 106 changes wavelengths of information beam and reference beam 111 for forming an interference pattern and a wavelength of a servo beam 112 for reading a servo pit. A first wavelength servo beam 112 passes between a transparent substrate 102 and a hologram recording layer 104. Information is recorded in the hologram recording layer 104 by the interference pattern. The filter layer 106 reflects a second wavelength information beam and the reference beam 111. In FIG. 7, servo pits are formed on the substrate 102. The substrate 102 is coated with gold, platinum, or the like. A reflection layer 103 is provided on the substrate 102. A transparent upper substrate 105 is provided on a front surface.
In Patent Literature 1, a combination of a layer formed from cholesteric liquid crystal and a ¼ wavelength plate layer or a dichroic mirror layer is proposed as the filter layer 106. The dichroic mirror layer is formed from a wavelength separating filter on which dielectric multilayer coating (sputtering) is performed.
When the recording medium 101 is used, the servo beam 112 passes through the filter layer 106 and is reflected by the reflection layer 103 on the front surface of the substrate 102. As a result, information in the servo pit can be read. After forming the interference pattern in the hologram recording layer 104, the information beam and the recording reference beam 111 irradiated during recording is reflected by the filter layer 106 without reaching the servo pit on the front surface of the substrate 102. Therefore, recording of unnecessary interference patterns caused by diffusion and reflection in the servo pit can be prevented. A reproduction reference beam 111 irradiated during reproduction is reflected by the filter layer 106 without reading the servo pit on the front surface of the substrate 102. Therefore, generation of noise caused by diffusion and reflection in the servo pit can be prevented.    Patent Literature 1: Japanese Patent Laid-open Publication No. 2004-265472    Patent Literature 2: Japanese Patent Laid-open Publication No. Heisei 11-311936